Abstract A highly crystalline copper(II) benzenehexathiolate coordination polymer (Cu‐BHT) has been prepared. The two‐dimensional kagome structure has been confirmed by powder X‐ray diffraction, high‐resolution transmission electron microscopy, and high‐resolution scanning transmission electron microscopy. The as‐prepared sample exhibits bulk superconductivity at about 0.25 K, which is confirmed by the zero resistivity, AC magnetic susceptibility, and specific heat measurements. Another diamagnetic transition at about 3 K suggests that there is a second superconducting phase that may be associated with a single layer or few layers of Cu‐BHT. It is the first time that superconductivity has been observed in a coordination polymer.
Single crystals of ${\mathrm{HfTe}}_{3}$ were successfully grown using a chemical transport reaction in an extremely narrow temperature range. Here, we report a comparative study of polycrystalline and single-crystal samples. The electrical resistivity $\ensuremath{\rho}(T)$ measured on polycrystalline samples shows a broad hump and clear drop at 80 and 1.7 K, which correspond to the formation of the charge density wave (CDW) and superconducting (SC) transition, respectively. For the single crystals, $\ensuremath{\rho}(T)$ shows a sharp change at ${T}_{\mathrm{CDW}}=93$ K, and the superconductivity is absent, in contrast to the polycrystalline samples. With the current flowing along the $a$ and $b$ directions, the coincidence of the linear temperature dependence of $\ensuremath{\rho}(T)/\ensuremath{\rho}(300\phantom{\rule{4pt}{0ex}}\mathrm{K})$ above ${T}_{\mathrm{CDW}}$ strongly implies that the electron-electron scattering mechanism dominates the transport properties in a quasi-one-dimensional chain. Furthermore, a metal-semiconductor-like transition is confirmed below ${T}_{\mathrm{CDW}}$ in ${\ensuremath{\rho}}_{c}$. The drop observed at 4.3 K in ${\ensuremath{\rho}}_{b}(T)$ for the single crystal with more defects (small residual resistivity ratio, large ${\ensuremath{\rho}}_{0}$, and weak drop) provides direct evidence of a disorder-related SC fluctuation in the CDW system. With temperature decreasing, the carrier density exhibits a similar and rapid decrease below ${T}_{\mathrm{CDW}}$ for flowing current in both the $a$ and $b$ directions, whereas an obvious enhancement of carrier mobility appears as $I\ensuremath{\parallel}b$. An analysis of x-ray photoelectron spectroscopy spectra suggests that the mixed-valence states of Hf and Te could be related to the CDW formation in the multichain system of ${\mathrm{HfTe}}_{3}$.
Abstract Ca(Al 1‐x Si x ) 2 (0.15 ≤ x ≤ 0.75) phases are prepared by arc melting of CaSi 2 and Al followed by RF induction heating (1000—1200 °C, 3 h).
Since the advent of graphene ushered the era of 2D materials, many forms of hydrogenated graphene have been reported, exhibiting diverse properties ranging from a tunable bandgap to ferromagnetic ordering. Patterned hydrogenated graphene with micron-scale patterns has been fabricated by lithographic means. Here, successful millimeter-scale synthesis of an intrinsically honeycomb-patterned form of hydrogenated graphene on Ru(0001) by epitaxial growth followed by hydrogenation is reported. Combining scanning tunneling microscopy observations with density-functional-theory (DFT) calculations, it is revealed that an atomic-hydrogen layer intercalates between graphene and Ru(0001). The result is a hydrogen honeycomb structure that serves as a template for the final hydrogenation, which converts the graphene into graphane only over the template, yielding honeycomb-patterned hydrogenated graphene (HPHG). In effect, HPHG is a form of patterned graphane. DFT calculations find that the unhydrogenated graphene regions embedded in the patterned graphane exhibit spin-polarized edge states. This type of growth mechanism provides a new pathway for the fabrication of intrinsically patterned graphene-based materials.
Recently, as a novel technique, electronic double-layer transistors (EDLTs) with ionic liquids have shown strong potential for tuning the electronic states of correlated systems. EDLT induced local carrier doping can always lead to dramatic changes in physical properties when compared to parent materials, e.g., insulating-superconducting transition. Generally, the modification of gate voltage ($V_{\rm G}$) in EDLT devices produces a direct change on the doping level, whereas the processing temperature ($T_{\rm G}$) at which $V_{\rm G}$ is applied only assists the tuning process. Here, we report that the processing temperature $T_{\rm G}$ plays a dominant role in the electric field induced superconductivity in layered ZrNBr. When applying $V_{\rm G}$ at $T_{\rm G}\geq$ 250 K, the induced superconducting (SC) state permanently remains in the material, which is confirmed in the zero resistance and diamagnetism after long-time relaxation at room temperature and/or by applying reverse voltage, whereas the solid/liquid interface induced reversible insulating-SC transition occurs at $T_{\rm G}\leq$ 235 K. These experimental facts support another electrochemical mechanism that electric field induced partial deintercalation of Br ions could cause permanent electron doping to the system. Our findings in this study will extend the potential of electric fields for tuning bulk electronic states in low-dimension systems.
The Mott insulator provides an excellent foundation for exploring a wide range of strongly correlated physical phenomena, such as high-temperature superconductivity, quantum spin liquid, and colossal magnetoresistance. A Mott insulator with the simplest degree of freedom is an ideal and highly desirable system for studying the fundamental physics of Mottness. In this study, we have unambiguously identified such an anticipated Mott insulator in a van der Waals layered compound Nb3Cl8. In the high-temperature phase, where interlayer coupling is negligible, density functional theory calculations for the monolayer of Nb3Cl8 suggest a half-filled flat band at the Fermi level, whereas angle-resolved photoemission spectroscopy experiments observe a large gap. This observation is perfectly reproduced by dynamical mean-field theory calculations considering strong electron correlations, indicating a correlation-driven Mott insulator state. Since this half-filled band derived from a single 2a1 orbital is isolated from all other bands, the monolayer of Nb3Cl8 is an ideal realization of the celebrated single-band Hubbard model. Upon decreasing the temperature, the bulk system undergoes a phase transition, where structural changes significantly enhance the interlayer coupling. This results in a bonding-antibonding splitting in the Hubbard bands, while the Mott gap remains dominant. Our discovery provides a simple and seminal model system for investigating Mott physics and other emerging correlated states.
We succeeded in synthesizing a new cubic intermetallic compound PrCu 4 Ag in a fcc structure. Measurements of X-ray diffraction, magnetic susceptibility, magnetization, specific heat, electrical resistivity, thermal expansion, and elastic constants have been performed on single crystals of PrCu 4 Ag. A maximum value of χ( T ) with a corresponding peak in C ( T ) suggests that an antiferromagnetic phase transition occurs at T N = 2.4 K, where a sudden decrease in ρ 4 f ( T ) and a sharp peak in the thermal expansion coefficient α( T ) were observed. Characteristic Curie-type softening was observed in the temperature dependence of the transverse mode for ( C 11 - C 12 )/2 and C 44 from 70 K down to T N , which implies that the crystalline electric field (CEF) ground state is the magnetic triplet Γ 5 . The anisotropic properties in M ( T , H ) and C ( T , H ) are studied when the external magnetic field is applied in the <100>, <110>, and <111> directions. Another anomaly observed at approximately 10 K in χ( T ) is considered to be related to possible disorder behavior. The low- T behavior and the possibility of quadrupole fluctuation are discussed with respect to the CEF effect. All the experimental results in the present study are summarized in a magnetic phase diagram.
The Zintl compound CaAl2Si2 peritectically decomposes to a new ternary cubic Laves phase Ca(Al,Si)2 and an Al–Si eutectic at temperatures above 750 °C under a pressure of 13 GPa. The ternary Laves phase compound can also be prepared as solid solutions Ca(Al1–xSix)2 (0.35 ≤ x ≤ 0.75) directly from the ternary mixtures under high-pressure and high-temperature conditions. The cubic Laves phase structure can be regarded as a type of clathrate compound composed of face-sharing truncated tetrahedral cages with Ca atoms at the center, Ca@(Al,Si)12. The compound with a stoichiometric composition CaAlSi exhibits superconductivity with a transition temperature of 2.6 K. This is the first superconducting Laves phase compound composed solely of commonly found elements.
We report the synthesis and characterization of a new ternary molybdenum pnictide superconductor, Cs2Mo3As3. The powder x-ray diffraction analysis reveals the quasi-one-dimensional (Q1D) hexagonal crystal structure formed by Cs+ and infinite (Mo3As3)2− chains as indicated by the wire-like grain morphology. Electrical resistivity and magnetic susceptibility characterizations exhibit superconductivity with the onset transition temperature at 11.5 K, which is the highest in all Q1D superconductors reported so far. An upper critical magnetic field of about 61.7 T at zero temperature was extrapolated from the resistivity measurement under a magnetic field, which is much higher than the Pauli paramagnetic limit, and the reason for such a high upper critical field may lie in its unconventional nature of superconducting pairing symmetry. The discovery of Cs2Mo3As3 inspires the search for new superconductors for future high field applications.
We systematically investigate the magnetization and thermodynamic responses associated with the antiferromagnetic (AFM) transitions in magnetic semiconductor Eu3InAs3. The linear thermal expansion measurements reveal thataaxis expands whereasbandcaxes contract with the onset of the two AFM transitions atTN1andTN2. Using a simplified mean-field model incorporating AFM exchange interactions, easy-axis anisotropy, and Zeeman coupling, we analyze the potential magnetic structure change associated with the spin-flop and spin-flip transitions in field. The agreement between experimental and calculated magnetization data suggests that the1/3plateau alongbaxis results from a partial spin-flip transition in a multiple-easy-axis magnetic structure, where Eu2-Eu3and Eu1sublattices order antiferromagnetically along thebandaaxes atTN1andTN2, respectively. Consistently, field dependence of magnetic entropy determined using low-Tadiabatic magnetocaloric effect indicates that the number of the ordered Eu2+moments atTN1is nearly twice that atTN2. Our results demonstrate that the magnetic structure in materials with strong spin-lattice coupling can be simply approached by a combined magnetization and thermodynamic study.
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